LibJS: Report string properties using UTF-16 code units

String length is reported as the number of UTF-16 code units, and string
indices are reported as the UTF-16 code units themselves.

Userland/Libraries/LibJS/Runtime/StringObject.cpp

@@ -5,7 +5,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/Utf8View.h>
+#include <AK/Utf16View.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/PrimitiveString.h>
@@ -34,7 +34,7 @@ void StringObject::initialize(GlobalObject& global_object)
 {
     auto& vm = this->vm();
     Object::initialize(global_object);
-    define_direct_property(vm.names.length, Value(Utf8View(m_string.string()).length()), 0);
+    define_direct_property(vm.names.length, Value(m_string.utf16_string_view().length_in_code_units()), 0);
 }
 
 void StringObject::visit_edges(Cell::Visitor& visitor)
@@ -70,17 +70,17 @@ static Optional<PropertyDescriptor> string_get_own_property(GlobalObject& global
 
     // 8. Let str be S.[[StringData]].
     // 9. Assert: Type(str) is String.
-    auto& str = string.primitive_string().string();
+    auto str = string.primitive_string().utf16_string_view();
 
     // 10. Let len be the length of str.
-    auto length = str.length();
+    auto length = str.length_in_code_units();
 
     // 11. If ℝ(index) < 0 or len ≤ ℝ(index), return undefined.
     if (index.as_double() < 0 || length <= index.as_double())
         return {};
 
     // 12. Let resultStr be the String value of length 1, containing one code unit from str, specifically the code unit at index ℝ(index).
-    auto result_str = js_string(string.vm(), str.substring(index.as_double(), 1));
+    auto result_str = js_string(string.vm(), str.substring_view(index.as_double(), 1));
 
     // 13. Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]: false, [[Enumerable]]: true, [[Configurable]]: false }.
     return PropertyDescriptor {
@@ -138,12 +138,12 @@ MarkedValueList StringObject::internal_own_property_keys() const
     auto keys = MarkedValueList { heap() };
 
     // 2. Let str be O.[[StringData]].
-    auto& str = m_string.string();
+    auto str = m_string.utf16_string_view();
 
     // 3. Assert: Type(str) is String.
 
     // 4. Let len be the length of str.
-    auto length = str.length();
+    auto length = str.length_in_code_units();
 
     // 5. For each integer i starting with 0 such that i < len, in ascending order, do
     for (size_t i = 0; i < length; ++i) {

Userland/Libraries/LibJS/Tests/builtins/String/String.js

@@ -13,4 +13,30 @@ test("length", () => {
     expect(new String("a").length).toBe(1);
     expect(new String("\u180E").length).toBe(1);
     expect(new String("\uDBFF\uDFFF").length).toBe(2);
+
+    // Issue #2280
+    expect("⛳".length).toBe(1);
+    expect("🔥".length).toBe(2);
+    expect("🔥🔥🔥".length).toBe(6);
+    expect("👨‍👩‍👦".length).toBe(8);
+    expect("👩‍❤️‍💋‍👩".length).toBe(11);
+});
+
+test("indices", () => {
+    expect("abc"[0]).toBe("a");
+    expect("abc"[1]).toBe("b");
+    expect("abc"[2]).toBe("c");
+    expect("abc"[3]).toBeUndefined();
+
+    expect("😀"[0]).toBe("\ud83d");
+    expect("😀"[1]).toBe("\ude00");
+    expect("😀"[2]).toBeUndefined();
+});
+
+test("properties", () => {
+    expect(Object.getOwnPropertyNames("")).toEqual(["length"]);
+    expect(Object.getOwnPropertyNames("a")).toEqual(["0", "length"]);
+    expect(Object.getOwnPropertyNames("ab")).toEqual(["0", "1", "length"]);
+    expect(Object.getOwnPropertyNames("abc")).toEqual(["0", "1", "2", "length"]);
+    expect(Object.getOwnPropertyNames("😀")).toEqual(["0", "1", "length"]);
 });